Conventional filter filtration material technologies (especially for fabric filter) operate as reverse flow cleaning systems or as online pulse jet cleaning systems. The reverse flow technology is rather old while the pulse jet technology is presently the most common.
The reverse flow technology normally separates a complete filter chamber, having 10% to 25% of the overall filtration area, from the filtering operation. Then, cleaning gas, which is often the gas from the clean gas side of the filter filtration material, is conveyed by means of a fan into the separated filter chamber to create a reverse flow through the filtration material. This technology requires an expensive and complex design of the filter system (e.g. big dampers for raw gas and clean gas) and requires more filtration material (for an increased filtration surface). Furthermore, the area of the part of the filter still in operation has to be increased due to the additional cleaning gas back flowing which has to be cleaned as well. Finally, the method is not very effective and provides only low operation safety (capability to cover changing filtration conditions). The advantage is, however, that no expensive compressed cleaning gas is required.
The pulse jet technology is presently the most common cleaning technology. It is a so-called online cleaning technology in which the filter segments to be cleaned are not separated from the gas stream. The cleaning is achieved by providing a high pressure gas impulse which causes a shock wave that sets the filtered particles (filter-cake) into vibration. The gas impulse used must be so strong that the filtered particles overcome the adherence to the filtration material caused by the gas stream still flowing through the filtration material; the efficiency of the vibration is not guaranteed. The disadvantages of this technology are, (1) that a huge amount of compressed cleaning gas is required, (2) that the shock waves cause a strong stress on the filtration material, which reduces the life time of the material, and (3) that—caused by the gas flow still flowing through the filtration material—the filtered particles are massaged into the filtration material, which increases the pressure drop of the filtration material up to 20 times. A further disadvantage is that cleaning with a filter-cake blast causes a stream of fine particles which return to the cleaned filter segment or to the one next to it. The aforementioned disadvantages reduce the efficiency of the technology and demand a low gas to filtration material ratio (that is: amount of gas flow per unit of time and per square meter). The advantages of this technology are, however, that no separation of the segments is necessary and that no complex ductwork for the cleaning gas is needed.